It's not a perfect book (slightly outdated, ugly HTML formatting) but still
a good read. The book starts from language fundamentals (types, functions, control structures, modules)
and works up to some interesting applications (spelling correcter, minesweeper clone, alpha-beta pruning). Also
covers more advanced topics like concurrency, when to use modules vs. objects, and using some of the associated OCaml tools.

Standard Tools

In addition to learning the language, it also helps to know about a few tools which will make your life easier.